The invention relates to objective lenses used in a variety of applications, including microlithography, photo-ablabon, ultraviolet (UV) inspection, and mastering of optical data storage media, such as CD and DVD.
Prior art objective lenses suffer from problems that render them inferior for our purposes. One type of prior art objective lens suitable for many applications is the double mirror objective lens, examples of which are shown in FIGS. 1A, 2A, and 3A, which are shown approximately to scale relative to one another. While relatively easy and inexpensive to manufacture, and while enjoying little, if any, chromatic aberration, double mirror objectives using two spherical mirrors must generally adhere to the Schwarzschild ratio, a ratio of the radii of curvature of the lenses equal to a value of about 2.6, to reduce aberrations. Adherence to the Schwarzschild ratio limits the amount by which obscuration can be reduced without undesirably increasing aberrations. In the example shown in FIG. 1A, the double mirror objective 1 includes a light path 2 along which light travels to strike a convex mirror 3, which reflects the light onto a concave mirror 4. In the objective lens of FIG. 1A, the light path 2 includes a hole in the concave mirror 4, but the light can be transmitted to the convex mirror 3 in other ways. The concave mirror 4 then reflects the light toward the convex mirror 3 and toward a focal point 5 of the objective 1, which is typically behind the convex mirror 3. This type of double mirror objective forms images with little or no chromatic aberrations, but suffers a large percentage of obscuration by the convex mirror 3 and has a relatively small maximum numerical aperture. When made with a relatively large working distance, the spherical aberrations throughout the image field are unacceptably large as shown in FIGS. 1B-1F. FIG. 2A shows an objective 1xe2x80x2 of the type shown in FIG. 1A, but made in relatively compact form to reduce spherical aberrations. While the spherical aberrations of the objective 1xe2x80x2 of FIG. 2A are significantly improved as seen in FIGS. 2B-2F, the objective 1xe2x80x2 has a much smaller working distance, still has a large obscuration, and still has a relatively small maximum numerical aperture. In FIGS. 2B-2F, traces for three wavelengths, xcex1, xcex2, xcex3, have been included to illustrate the lack of significant chromatic aberrations introduced by this type of objective. In the particular example, xcex1=248.6 nm, xcex2=248.4 nm, and xcex3=248.2 nm.
In applications requiring lower obscurations, the double mirror objective can be forced to have a smaller obscuration as shown in FIG. 3A. Here, the objective 1xe2x80x3 includes a convex mirror 3xe2x80x3 that is much smaller in relation to the light cone reflected from the concave mirror 4xe2x80x3, yielding a much lower obscuration. However, the reduced obscuration comes at the cost of high spherical aberrations, as seen in FIGS. 3B-3F. While these aberrations can be reduced using large departure aspheric mirror surfaces, such large departure surfaces are more difficult and costly to manufacture. As in FIGS. 2B-2F, traces for three wavelengths, xcex1, xcex2, xcex3, have been included in FIGS. 3B-3F to illustrate the lack of significant chromatic aberrations introduced by this type of objective. In the particular example, xcex1=248.6 nm, xcex2=248.4 nm, and xcex3=248.2 nm.
Adding a refractor to the double mirror objective overcomes the problem of spherical aberration in relatively low obscurabon double mirror objectives while avoiding the use of large departure aspheric surfaces. However, introducing such a refractor into the objective yields a catadioptric lens system, which suffers from chromatic aberrations. The chromatic aberrations limit the wavelength range or bandwidth with which a given prior art catadioptric objective can reasonably be used, making such systems impractical for many applications. In addition, prior art catadioptric objectives have problems related to placement of the convex mirror that render the objectives insufficient for our needs.
A particularly good prior art catadioptric objective includes the convex mirror placed in front of a two optical surface refractor. See, for example, U.S. Pat. No. 2,520,635 to Grey. While this arrangement significantly reduces spherical aberration and suffers from a lower chromatic aberration than many other catadioptric objectives, it still suffers from an unacceptably large obscuration. Further, the numerical aperture of this type of objective is still limited in its range. In addition, the convex mirror must be glued or otherwise attached to the refractor, which can cause other problems. In particular, glue can foul the refractor and, in some cases, can break down if exposed to UV rays.
Another good prior art catadioptric objective, an optical disc reader objective disclosed in U.S. Pat. No. 4,835,380 to Opheij et al., uses a concave Mangin mirror with a convex mirror positioned at the center of the Mangin mirror""s front surface. This objective enjoys the reduced spherical aberrations of the catadioptric system, but can only be used with a narrow bandwidth of light. Here again, the convex mirror must be glued to or otherwise attached to or supported against the Mangin mirror""s front surface. While glues and adhesives work well for the wavelengths used in optical disc readers, many other applications, such as microlithography, use wavelengths that break down most adhesives in short order. In addition, adhesives can alter the light to some degree. Thus, gluing a convex mirror on the front of a Mangin mirror is not a viable solution in applications using wavelengths of light unfriendly to adhesives or that require a minimum of image degradation.
Thus, prior art double mirror objectives can enjoy reduced chromatic and spherical aberration when sized in accordance with the Schwarzschild ratio, but suffer from high obscurations and short working distances. Prior art catadioptric objectives enjoy lower obscurations and low spherical aberrations, but suffer from narrow usable bandwidths because of relatively high chromatic errors. Further, no prior art system has adequately low obscuration, adequately low spherical aberration, and a suitable working distance for many applications, let alone the additional feature of low chromatic aberration. All of these prior art objectives further suffer from relatively low numerical apertures.
We achieve superior performance in a catadioptric objective by moving the convex mirror to an opposite side of the refractor from the concave mirror, preferably without the use of adhesives or other supports. In the preferred embodiments, we use a multifunction component with a refractor portion and a reflector portion, the reflector portion being the convex mirror and being formed in a surface of the refractor portion. We prefer to grind a depression into a back or right surface of the refractor portion so that the periphery of the reflector portion is surrounded by the back or right refractive interface of the refractor portion. We then polish the depression and coat it with a coating that renders the depression reflective into the refractive material of the refractor portion and toward the concave reflector. In this manner, we produce a multifunction component that retracts, reflects, and again refracts light in one part of a light path through the objective, then refracts the light twice in another part of the light path. For example, in an objective where the light enters the system and crosses the front or left refractive interface of the refractor portion, the light is refracted as it crosses the refractive interface, travels through the refractor portion to reflect from the convex mirror, travels across the front or left refractive interface of the refractor portion where it is again refracted. The light then travels to the concave mirror, where it reflects back toward the multifunction component, at which point the light is refracted twice as it crosses the left and right refractive interfaces of the outer portion of the refractor portion. This preferred embodiment yields an objective with very little spherical aberration over a relatively narrow bandwidth. In a variation of this preferred embodiment, we form the concave mirror as a Mangin mirror and orient the Mangin mirror so that the surface not carrying the reflective coating is an additional refractive interface in the objective. This additional refractive interface corrects chromatic errors, yielding an objective with low spherical and chromatic aberrations that is useful over a relatively wide bandwidth.
A major benefit of the invention is that it can be used with other optics on either end of the objective or even inserted between the concave mirror and the multifunction component. When used with other components in this manner, the concave mirror and multifunction component can greatly reduce spherical and chromatic errors for particular applications, such as those discussed above. Thus, the invenbon can be applied in a great many ways and with a great many variations.
In a variation of the exemplary embodiment, we form the concave mirror as a Mangin mirror, effectively creating a second multifunction component. The left surface of the Mangin mirror preferably carries the reflective coating so that the right surface of the Mangin mirror is an additional refractive interface. This additional refractive interface corrects chromatic errors introduced by the concave mirror, then reflects from the concave mirror to the refractor so that the light refracts through the first and second surfaces of the refractor as it travels to a second end of the system. Such an arrangement yields an objective that suffers from little spherical aberration, enjoys low obscuration, has a higher numerical aperture, and has a relatively large working distance, overcoming most of the deficiencies of prior art objectives.
We have also found that forming the concave mirror as a Mangin mirror not only further reduces the spherical aberration caused by the concave mirror, but also dramatically reduces the chromatic aberration of the system. Thus, this form of our inventive catadioptric objective overcomes even more of the deficiencies of the prior art and is suitable for a wide variety of applications.
In our preferred embodiment, we grind and polish a depression in the right surface of the refractor, coating the depression with a reflective coating, if necessary, to form the convex mirror. This overcomes the problems in the prior art associated with supporting the convex mirror, reduces obscuration by moving the convex mirror back, and allows greater correction for spherical aberration since light incident on the convex mirror is refracted on its way to the mirror and on its way out of the refractor.